%) compared to that in basalt with less than 52 wt. However, the andezit has higher SiO2 content (>ĥ2 wt. ClassificationĪccording to modal composition projected within the QAPFĭiscrimination diagram for volcanic rock (Streckeisen, 1978), the andesite Major minerals of Andesite: Plagioclase, hornblende, almandine.Īccessory minerals of Andesite:Ilmenite, a patite and orthopyroxene. To porphyric with redish phenocrysts of garnet and plagioclase.Īre in places transformed to clay minerals. Name is after Andes – the mountain chain extending along the western coast ofīut typically bluish-grey or grey (lighter coloured than basalt). Ancient andesites are used to map ancient subduction zones because andesitic volcanoes form on continental or ocean crust above these zones. Andesite erupts from volcanoes and is commonly found interbedded with volcanic ash and tuff.
Amygdaloidal andesite occurs when the voids left by gas bubbles in the solidifying magma are later filled in, often with zeolite minerals.
It consists of the plagioclase feldspar minerals andesine and oligoclase, together with one or more dark, ferromagnesian minerals such as pyroxene and biotite. Andesite is the volcanic equivalent of diorite. Intermediate in silica content, it is usually gray in color and may be fine-grained or porphyritic.
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